i.MX6 Power: ARM LDO bypass?

[Rob_iMX6]

Hi everyone,

the i.MX6 comes with on chip LDO-voltage regulators for the ARM core, the SOC and many other sections. In our application (industrial control), the CPU and SOC will always run at 800 or 1000MHz, no DVFS, no sleep, 24hours/7days.

In order to save power and simplify layout, I consider to bypass the LDO of the ARM and connect the VDD_ARM_CAP with the input VDD_ARM_IN. Same with the SOC. Are there any reasons, why I should not do this? In the PMU unit, one can set the LDO to bypass (TARG=0x1F), see the RM at page 4481. External bypass is mentioned on the same page.

Kind regards!

-Rob

[JorgeRama_rezRi · Accepted solution]

Hi Rob,

Your assumption is correct. The voltage rails that have internal LDOs (core, PU, SoC) require very precise and accurate levels. An external switcher won't be able to provide such an accurate voltage due to its switching nature. Now, if you use an external LDO, the power dissipation on it will be a lot more, the internal LDOs were planned for a little voltage drop since the external switcher (which is more efficient) makes the tough work of lowering it from say 3.3V, that's why we recommend to use the internal ones, the power dissipation is really small on them. In an external LDO, it would have to drop from 3.3V to 1.225V, so, let's say the i.MX draws 1A from it, the power dissipation will be ~2W.

Hope this helps!

Best regards.

Jorge.

[Rob_iMX6]

Well, I think that I can answer my question by myself:

The advantage of having the LDOs on-chip is that they provide tighter control over the processor voltage: They are trimmed to high precision and can reject the voltage ripple on the power source.

If one can provide a very precise, low voltage ripple power supply, it seems to be possible to bypass the LDOs. Table 9 of the datasheet defines the required voltages, also for the "Run mode: LDO bypassed". Unfortunately, there is no spec on the allowed maximum voltage ripple.

The disadvantage of this LDO-on-chip concept is a more complex layout. A 6 layer PCB seems not to be possible.

Other ideas are still welcome!

Kind regards,

-Rob